Safely close database connections and delete backends

Also fix NewClosure leak caused by disconnected object signals

src/playlist/playlistdelegates.cpp

@@ -22,6 +22,7 @@
 #include "config.h"
 
 #include <QtGlobal>
+#include <QApplication>
 #include <QObject>
 #include <QWidget>
 #include <QtConcurrentRun>
@@ -363,6 +364,10 @@ TagCompletionModel::TagCompletionModel(CollectionBackend *backend, Playlist::Col
     setStringList(backend->GetAll(col));
   }
 
+  if (QThread::currentThread() != backend->thread() && QThread::currentThread() != qApp->thread()) {
+    backend->Close();
+  }
+
 }
 
 QString TagCompletionModel::database_column(Playlist::Column column) {